Method and System for Constraining Slice Header Processing Overhead in Video Coding

ABSTRACT

A method for encoding a picture of a video sequence in a bit stream that constrains slice header processing overhead is provided. The method includes computing a maximum slice rate for the video sequence, computing a maximum number of slices for the picture based on the maximum slice rate, and encoding the picture wherein a number of slices used to encode the picture is enforced to be no more than the maximum number of slices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/699,474, filed Sep. 11, 2012, and U.S. Provisional PatentApplication Ser. No. 61/704,648, filed Sep. 24, 2012, both of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to constrainingslice header processing overhead in video coding.

2. Description of the Related Art

The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T WP3/16and ISO/IEC JTC 1/SC 29/WG 11 is currently developing thenext-generation video coding standard referred to as High EfficiencyVideo Coding (HEVC). Similar to previous video coding standards such asH.264/AVC, HEVC is based on a hybrid coding scheme using block-basedprediction and transform coding. First, the input signal is split intorectangular blocks that are predicted from the previously decoded databy either motion compensated (inter) prediction or intra prediction. Theresulting prediction error is coded by applying block transforms basedon an integer approximation of the discrete cosine transform, which isfollowed by quantization and coding of the transform coefficients.

Similar to prior video coding standards, a picture may be subdividedinto one or more independently decodable slices. A slice header istransmitted for each slice. This header contains high-level parametersfor decoding the data in the associated slice. Such parameters mayinclude, for example, a picture order counter, reference pictureparameter sets, SAO (sample adaptive offset) control parameters,reference picture list parameters and modification parameters,de-blocking filter control parameters, a weighted prediction table,in-loop filter slice on/off control flag, and sub-stream entries, etc.

The processing overhead associated with a slice header includes sliceheader parsing, slice-level operations such as reference picture listre-ordering, and CABAC resets. In typical video decoder designs, sliceheader parsing and operations such as reference picture list re-orderingare implemented in software for flexibility and error resiliencyreasons. What software can do is fairly limited. Therefore, processingslice headers may introduce significant overhead for real-time decoderimplementations if a bit stream contains the maximum number of slicespermitted by the standard. Accordingly, constraining the overhead ofslice header parsing is desirable.

SUMMARY

Embodiments of the present invention relate to methods, apparatus, andcomputer readable media that constrain slide header processing overheadas compared to the prior art. In one aspect, a method for encoding apicture of a video sequence in a bit stream is provided that includescomputing a maximum slice rate for the video sequence, computing amaximum number of slices for the picture based on the maximum slicerate, and encoding the picture wherein a number of slices used to encodethe picture is enforced to be no more than the maximum number of slices.

In one aspect, a method for encoding a picture of a video sequence in abit stream is provided that includes determining a level for the videosequence, computing a maximum slice rate for the video sequence asMaxLumaSR/MaxLumaPS*MaxSlicesPerPicture, wherein MaxLumaSR is a maximumluma sample rate specified for the level, MaxLumaPS is a maximum lumapicture size in samples specified for the level, and MaxSlicesPerPictureis a maximum number of slices per picture specified for the level,computing a maximum number of slices for the picture as a minimum ofMaxSlicesPerPicture and the maximum slice rate divided by a frame rateof the video sequence when the frame rate is fixed, computing a maximumnumber of slices for the picture as a minimum of MaxSlicesPerPicture andthe maximum slice rate multiplied by a difference in display timebetween the picture and a picture immediately preceding the picture indisplay order when the frame rate is variable, and encoding the picturewherein a number of slices used to encode the picture is constrained tobe no more than the maximum number of slices.

In one aspect, an apparatus configured to encode a picture of a videosequence in a bit stream is provided that includes means for computing amaximum slice rate for the video sequence, means for computing a maximumnumber of slices for the picture based on the maximum slice rate, andmeans for encoding the picture wherein a number of slices used to encodethe picture is enforced to be no more than the maximum number of slices.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments will now be described, by way of example only,and with reference to the accompanying drawings:

FIG. 1 is an example level table for a profile;

FIG. 2 is a block diagram of a digital system;

FIGS. 3A and 3B are a block diagram of an example video encoder;

FIG. 4 is a block diagram of an example video decoder;

FIGS. 5 and 6 are flow diagrams of methods;

FIG. 7 is an example variable frame rate sequence; and

FIG. 8 is a block diagram of an illustrative digital system.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency.

As used herein, the term “picture” may refer to a frame or a field of aframe. A frame is a complete image captured during a known timeinterval. For convenience of description, embodiments are describedherein in reference to HEVC. One of ordinary skill in the art willunderstand that embodiments of the invention are not limited to HEVC.

In HEVC, a largest coding unit (LCU) is the base unit used forblock-based coding. A picture is divided into non-overlapping LCUs. Thatis, an LCU plays a similar role in coding as the macroblock ofH.264/AVC, but it may be larger, e.g., 32×32, 64×64, etc. An LCU may bepartitioned into coding units (CU). A CU is a block of pixels within anLCU and the CUs within an LCU may be of different sizes. Thepartitioning is a recursive quadtree partitioning. The quadtree is splitaccording to various criteria until a leaf is reached, which is referredto as the coding node or coding unit. The maximum hierarchical depth ofthe quadtree is determined by the size of the smallest CU (SCU)permitted. The coding node is the root node of two trees, a predictiontree and a transform tree. A prediction tree specifies the position andsize of prediction units (PU) for a coding unit. A transform treespecifies the position and size of transform units (TU) for a codingunit. A transform unit may not be larger than a coding unit and the sizeof a transform unit may be, for example, 4×4, 8×8, 16×16, and 32×32. Thesizes of the transforms units and prediction units for a CU aredetermined by the video encoder during prediction based on minimizationof rate/distortion costs.

Various versions of HEVC are described in the following documents, whichare incorporated by reference herein: T. Wiegand, et al., “WD3: WorkingDraft 3 of High-Efficiency Video Coding,” JCTVC-E603, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11, Geneva, CH, Mar. 16-23, 2011 (“WD3”), B. Bross,et al., “WD4: Working Draft 4 of High-Efficiency Video Coding,”JCTVC-F803_d6, Joint Collaborative Team on Video Coding (JCT-VC) ofITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Torino, IT, Jul. 14-22, 2011(“WD4”), B. Bross. et al., “WD5: Working Draft 5 of High-EfficiencyVideo Coding,” JCTVC-G1103_d9, Joint Collaborative Team on Video Coding(JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Geneva, CH, Nov.21-30, 2011 (“WD5”), B. Bross, et al., “High Efficiency Video Coding(HEVC) Text Specification Draft 6,38 JCTVC-H1003_dK, Joint CollaborativeTeam on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IECJTC1/SC29/WG1, San Jose, Calif., Feb. 1-10, 2012, (“HEVC Draft 6”), B.Bross, et al., “High Efficiency Video Coding (HEVC) Text SpecificationDraft 7,” JCTVC-11003_d9, Joint Collaborative Team on Video Coding(JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1, Geneva, CH, Apr.17-May 7, 2012 (“HEVC Draft 6”), B. Bross, et al., “High EfficiencyVideo Coding (HEVC) Text Specification Draft 8,” JCTVC-J1003_d7, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG1, Stockholm, SE, Jul. 11-20, 2012 (“HEVC Draft 8”),B. Bross, et al., “High Efficiency Video Coding (HEVC) TextSpecification Draft 9,” JCTVC-K1003_v13, Joint Collaborative Team onVideo Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1,Shanghai, CN, Oct. 10-19, 2012 (“HEVC Draft 9”), and B. Bross, et al.,“High Efficiency Video Coding (HEVC) Text Specification Draft 10 (forFDIS & Last Call),” JCTVC-L1003_v34, Joint Collaborative Team on VideoCoding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1, Geneva, CH,Jan. 14-23, 2013 (“HEVC Draft 10”).

Some aspects of this disclosure have been presented to the JCT-VC in M.Zhou, “AHG9: On Number of Slices Constraint,” JCTVC-K0201, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG1, Shanghai, China, Oct. 10-19, 2012, which isincorporated by reference herein in its entirety.

As previously mentioned, constraining the overhead of parsing sliceheaders is desirable. In the prior art HEVC specification, HEVC Draft 8,to address the slice header processing overhead issue, in bitstreamsconforming to the Main profile of HEVC, the number of slices in apicture is restricted to be is less than or equal to a maximum number ofslices, referred to as MaxSlicesPerPicture. The value ofMaxSlicesPerPicture is level dependent and is specified in Table A-1 ofHEVC Draft 8. This table is replicated in FIG. 1. In HEVC (and othervideo coding standards), a profile specifies a set of coding tools thatare supported by the profile, and a level specifies parameterconstraints such as maximum sample rate, maximum bit-rate, maximumpicture size, etc.

For any given level, a complaint decoder may need to support real-timedecoding of video bit streams with different frame-rates and/or picturesizes. For example, level 5 in the table of FIG. 1 supports 4K×2K@30(4K×2K pictures at 30 frames per second) and the maximum number ofslices per second is 6000 slices/sec, i.e., 30 frames/sec multiplied by200 slices/frame. For the same maximum sample rate constraint of level5, a level 5 compliant decoder also needs to be able to decode video bitstreams at, for example, 1080p@120 and 720p@240, in real-time. Becauseonly the maximum number of slices per picture is fixed (200 per picturein this case), the worst case number of slices per second increases to24000 slices/sec, i.e., 120 frames/sec multiplied by 200 slices/frame,and 48000 slices/sec, i.e., 240 frames/sec multiplied by 200slices/frame, for 1080p@120 and 720p@240, respectively. Thus, the worstcase slice header processing overhead can vary for a level based on theframe rate of the incoming compressed video bit stream, which imposes ahuge burden for designing a real-time decoder which is required to dealwith worst cases. Therefore, merely limiting the maximum number ofslices per picture is not sufficient for constraining slice headerprocessing overhead, because the overhead increases proportionally tothe frame-rate of a coded sequence.

Embodiments of the invention provide for constraining the worst casenumber of slices per second rate to be constant for a level. The worstcase number of slices per second rate (MaxSliceRate) for a level may bedefined as

MaxSliceRate=MaxLumaSR/MaxLumaPS*MaxSlicesPerPicture

where MaxLumaSR is the maximum luma sample rate, MaxLumaPS is themaximum luma picture size in samples, and MaxSlicesPerPicture is themaximum number of slices in a picture. Example values of MaxLumaSR,MaxLumaPS and MaxSlicesPerPicture for each level are shown in theexample level table of FIG. 1. One of ordinary skill in the art willunderstand embodiments in which other suitable values are used and/or inwhich more or fewer levels are specified for a profile. One of ordinaryskill in the art will also understand that the parameters used tocompute the maximum slice rate may differ depending upon the particularparameters defined for levels.

FIG. 2 shows a block diagram of a digital system that includes a sourcedigital system 200 that transmits encoded video sequences to adestination digital system 202 via a communication channel 216. Thesource digital system 200 includes a video capture component 204, avideo encoder component 206, and a transmitter component 208. The videocapture component 204 is configured to provide a video sequence to beencoded by the video encoder component 206. The video capture component204 may be, for example, a video camera, a video archive, or a videofeed from a video content provider. In some embodiments, the videocapture component 204 may generate computer graphics as the videosequence, or a combination of live video, archived video, and/orcomputer-generated video.

The video encoder component 206 receives a video sequence from the videocapture component 204 and encodes it for transmission by the transmittercomponent 208. The video encoder component 206 receives the videosequence from the video capture component 204 as a sequence of pictures,divides the pictures into largest coding units (LCUs), and encodes thevideo data in the LCUs. The video encoder component 206 may beconfigured to comply with a worst case number of slices per second rateduring the encoding process as described herein. An embodiment of thevideo encoder component 206 is described in more detail herein inreference to FIGS. 3A and 3B.

The transmitter component 208 transmits the encoded video data to thedestination digital system 202 via the communication channel 216. Thecommunication channel 216 may be any communication medium, orcombination of communication media suitable for transmission of theencoded video sequence, such as, for example, wired or wirelesscommunication media, a local area network, or a wide area network.

The destination digital system 202 includes a receiver component 210, avideo decoder component 212 and a display component 214. The receivercomponent 210 receives the encoded video data from the source digitalsystem 200 via the communication channel 216 and provides the encodedvideo data to the video decoder component 212 for decoding. The videodecoder component 212 reverses the encoding process performed by thevideo encoder component 206 to reconstruct the LCUs of the videosequence. The video decoder component 212 may be configured to confirmcompliance with a worst case number of slices per second rate during thedecoding process as described herein. An embodiment of the video decodercomponent 212 is described in more detail below in reference to FIG. 4.

The reconstructed video sequence is displayed on the display component214. The display component 214 may be any suitable display device suchas, for example, a plasma display, a liquid crystal display (LCD), alight emitting diode (LED) display, etc.

In some embodiments, the source digital system 200 may also include areceiver component and a video decoder component and/or the destinationdigital system 202 may include a transmitter component and a videoencoder component for transmission of video sequences both directionsfor video streaming, video broadcasting, and video telephony. Further,the video encoder component 206 and the video decoder component 212 mayperform encoding and decoding in accordance with one or more videocompression standards. The video encoder component 206 and the videodecoder component 212 may be implemented in any suitable combination ofsoftware, firmware, and hardware, such as, for example, one or moredigital signal processors (DSPs), microprocessors, discrete logic,application specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), etc.

FIGS. 3A and 3B show block diagrams of an example video encoderconfigured to constrain the slice rate per picture during the encodingprocess to be less than or equal to a worst case number of slices persecond rate. FIG. 3A shows a high level block diagram of the videoencoder and FIG. 3B shows a block diagram of the LCU processingcomponent 642 of the video encoder. As shown in FIG. 3A, the videoencoder includes a coding control component 340, an LCU processingcomponent 342, and a memory 346. The memory 346 may be internal(on-chip) memory, external (off-chip) memory, or a combination thereof.The memory 346 may be used to communicate information between thevarious components of the video encoder.

An input digital video sequence is provided to the coding controlcomponent 340. The coding control component 340 sequences the variousoperations of the video encoder, i.e., the coding control component 340runs the main control loop for video encoding. For example, the codingcontrol component 340 performs processing on the input video sequencethat is to be done at the picture level, such as determining the codingtype (I, P, or B) of a picture based on a high level coding structure,e.g., IPPP, IBBP, hierarchical-B, and dividing a picture into LCUs forfurther processing.

In addition, for pipelined architectures in which multiple LCUs may beprocessed concurrently in different components of the LCU processing,the coding control component 340 controls the processing of the LCUs byvarious components of the LCU processing in a pipeline fashion. Forexample, in many embedded systems supporting video processing, there maybe one master processor and one or more slave processing modules, e.g.,hardware accelerators. The master processor operates as the codingcontrol component and runs the main control loop for video encoding, andthe slave processing modules are employed to off load certaincompute-intensive tasks of video encoding such as motion estimation,motion compensation, intra prediction mode estimation, transformationand quantization, entropy coding, and loop filtering. The slaveprocessing modules are controlled in a pipeline fashion by the masterprocessor such that the slave processing modules operate on differentLCUs of a picture at any given time. That is, the slave processingmodules are executed in parallel, each processing its respective LCUwhile data movement from one processor to another is serial.

The coding control component 340 also determines the profile and levelwithin the profile for the video sequence to be encoded. Typically, anencoder is designed for a particular profile. For example, HEVCcurrently defines two profiles: the Main profile for 8-bit video and theMain-10 profile for 8-bit and 10-bit video. If the encoder is designedfor only the Main profile, then the profile of any incoming 8-bit videowill be the Main profile. If the encoder is designed for the Main-10profile, then the profile of an incoming video sequence is set based onthe bit-depth of the video. That is, if the video is 8-bit video, thenthe profile will be the Main profile, and if the video is 10-bit video,then the profile will be the Main-10 profile. Typically, the encoderselects a level from the profile that is the minimum level satisfyingthe picture size, frame rate, bit-rate, etc. of the incoming videosequence as the desired result is to select a level that communicates toa decoder the minimum capability needed to decode the resultingcompressed bit stream.

Once the profile and level are determined, the coding control component340 computes a maximum slice rate (maximum number of slices per second)for pictures in the video sequence based on parameter values defined forthe level. Computation of the maximum slice rate is previously discussedherein.

The coding control component 340 then constrains the slice rate for eachpicture to be less than or equal to the maximum slice rate. In someembodiments, the coding control component 340 constrains the slice rateby computing a maximum number of slices for each picture based on themaximum slice rate constraint and then restricting the number of slicesused in encoding of the picture to be less than or equal to the computedmaximum slice number. More specifically, for each fixed frame ratepicture, the coding control component 340 computes the maximum number ofslices for the picture as

min(MaxSlicesPerPicture, MaxSliceRate/FrameRate)

where the value of MaxSliceRate is the maximum slice rate computed aspreviously described, and min(a, b) returns the minimum value of a andb. As previously mentioned, MaxSlicesPerPicture is the maximum number ofslices in a picture. Note that a picture should contain at least oneslice, thus the minimum value of the maximum number of slices in apicture is one.

For each variable frame rate picture, the coding control component 340computes the maximum number of slices for the picture as

min(MaxSlicesPerPicture, MaxSliceRate*(t(n)−t(n−1))

where t(n) is the display time of picture n in display order. For thefirst picture in the sequence (i.e., n=0), the initial display time,i.e., t(−1), can be determined by the initial specified delays. FIG. 7shows an example variable frame rate sequence.

The operation of the coding control component 340 to constrain thenumber of slices in a picture to be less than or equal to the computedmaximum number of slices for the picture may be performed in anysuitable way and may depend on the application using the video encoder.There are many techniques for constraining the number of slices in apicture currently known in the art. Note that a slice is some integernumber of LCUs. For example, in some video conferencing applications,each packet is limited to some number of bytes, e.g., 1500. To avoidpacket fragmentation, a packet normally contains a slice; thus thenumber of bits in a slice is capped at the packet size limit. In thiscase, the coding control component 340 tracks the total number ofconsumed bits of a current slice after encoding the current LCU. If thetotal number of bits exceeds the slice cap, the coding control component340 starts a new slice and causes the current LCU to be encoded again asthe first LCU of the new slice. In addition, the coding controlcomponent 340 monitors the total slice count for a picture. If the totalnumber of slices inserted into the current picture is approaching thecomputed maximum number of slices for the picture, the coding controlcomponent 340 may cause the remainder of LCUs in the picture to be morecoarsely quantized such that a slice can contain more LCUs within theslice cap limit, and the total number of slices for the picture wouldnot exceed the maximum.

In another example, the coding control component 340 may select a totalnumber of slices N for the current picture to be less than or equal tothe computed maximum number of slices for that picture and cause thepicture to be encoded into N slices, each with the approximately thesame number of number of LCUs. When a current slice contains thepre-determined number of LCUs, the coding control component 340 causesthe current slice to terminate and starts a new slice.

FIG. 3B is a block diagram of the LCU processing component 342. The LCUprocessing component 342 receives LCUs 300 of the input video sequencefrom the coding control component and encodes the LCUs 300 under thecontrol of the coding control component 340 to generate the compressedvideo stream. The LCUs 300 in each picture are processed in row order.The LCUs 300 from the coding control component are provided as one inputof a motion estimation component (ME) 320, as one input of anintra-prediction estimation component (IPE) 324, and to a positive inputof a combiner 302 (e.g., adder or subtractor or the like). Further,although not specifically shown, the prediction mode of each picture asselected by the coding control component 340 is provided to a modedecision component 328 and the entropy coding component 336.

The storage component 318 provides reference data to the motionestimation component 320 and to the motion compensation component 322.The reference data may include one or more previously encoded anddecoded pictures, i.e., reference pictures. Both list 0 and list 1reference pictures may be stored by the storage component 318.

The motion estimation component 320 provides motion data information tothe motion compensation component 322 and the entropy coding component336. More specifically, the motion estimation component 320 performstests on CUs in an LCU based on multiple inter-prediction modes (e.g.,skip mode, merge mode, and normal or direct inter-prediction), PU sizes,and TU sizes using reference picture data from storage 318 to choose thebest CU partitioning, PU/TU partitioning, inter-prediction modes, motionvectors, etc. based on coding cost, e.g., a rate distortion coding cost.To perform the tests, the motion estimation component 320 may divide anLCU into CUs according to the maximum hierarchical depth of thequadtree, and divide each CU into PUs according to the unit sizes of theinter-prediction modes and into TUs according to the transform unitsizes, and calculate the coding costs for each PU size, prediction mode,and transform unit size for each CU. The motion estimation component 320provides the motion vector (MV) or vectors and the prediction mode foreach PU in the selected CU partitioning to the motion compensationcomponent (MC) 322.

The motion compensation component 322 receives the selectedinter-prediction mode and mode-related information from the motionestimation component 320 and generates the inter-predicted CUs. Theinter-predicted CUs are provided to the mode decision component 328along with the selected inter-prediction modes for the inter-predictedPUs and corresponding TU sizes for the selected CU/PU/TU partitioning.The coding costs of the inter-predicted CUs are also provided to themode decision component 328.

The intra-prediction estimation component 324 (IPE) performsintra-prediction estimation in which tests on CUs in an LCU based onmultiple intra-prediction modes, PU sizes, and TU sizes are performedusing reconstructed data from previously encoded neighboring CUs storedin a buffer (not shown) to choose the best CU partitioning, PU/TUpartitioning, and intra-prediction modes based on coding cost, e.g., arate distortion coding cost. To perform the tests, the intra-predictionestimation component 324 may divide an LCU into CUs according to themaximum hierarchical depth of the quadtree, and divide each CU into PUsaccording to the unit sizes of the intra-prediction modes and into TUsaccording to the transform unit sizes, and calculate the coding costsfor each PU size, prediction mode, and transform unit size for each PU.The intra-prediction estimation component 324 provides the selectedintra-prediction modes for the PUs and the corresponding TU sizes forthe selected CU partitioning to the intra-prediction component (IP) 326.The coding costs of the intra-predicted CUs are also provided to theintra-prediction component 326.

The intra-prediction component 326 (IP) receives intra-predictioninformation, e.g., the selected mode or modes for the PU(s), the PUsize, etc., from the intra-prediction estimation component 324 andgenerates the intra-predicted CUs. The intra-predicted CUs are providedto the mode decision component 328 along with the selectedintra-prediction modes for the intra-predicted PUs and corresponding TUsizes for the selected CU/PU/TU partitioning. The coding costs of theintra-predicted CUs are also provided to the mode decision component328.

The mode decision component 328 selects between intra-prediction of a CUand inter-prediction of a CU based on the intra-prediction coding costof the CU from the intra-prediction component 326, the inter-predictioncoding cost of the CU from the motion compensation component 322, andthe picture prediction mode provided by the coding control component.Based on the decision as to whether a CU is to be intra- or inter-coded,the intra-predicted PUs or inter-predicted PUs are selected. Theselected CU/PU/TU partitioning with corresponding modes and other moderelated prediction data (if any) such as motion vector(s) and referencepicture index (indices), are provided to the entropy coding component336.

The output of the mode decision component 328, i.e., the predicted PUs,is provided to a negative input of the combiner 302 and to the combiner338. The associated transform unit size is also provided to thetransform component 304. The combiner 302 subtracts a predicted PU fromthe original PU. Each resulting residual PU is a set of pixel differencevalues that quantify differences between pixel values of the original PUand the predicted PU. The residual blocks of all the PUs of a CU form aresidual CU for further processing.

The transform component 304 performs block transforms on the residualCUs to convert the residual pixel values to transform coefficients andprovides the transform coefficients to a quantize component 306. Morespecifically, the transform component 304 receives the transform unitsizes for the residual CU and applies transforms of the specified sizesto the CU to generate transform coefficients. Further, the quantizecomponent 306 quantizes the transform coefficients based on quantizationparameters (QPs) and quantization matrices provided by the codingcontrol component and the transform sizes and provides the quantizedtransform coefficients to the entropy coding component 336 for coding inthe bit stream.

The entropy coding component 336 entropy encodes the relevant data,i.e., syntax elements, output by the various encoding components and thecoding control component using context-adaptive binary arithmetic coding(CABAC) to generate the compressed video bit stream. Among the syntaxelements that are encoded are picture parameter sets, slice headers,flags indicating the CU/PU/TU partitioning of an LCU, the predictionmodes for the CUs, and the quantized transform coefficients for the CUs.The entropy coding component 336 also entropy encodes relevant data fromthe in-loop filters, such as the SAO parameters.

The LCU processing includes an embedded decoder. As any compliantdecoder is expected to reconstruct an image from a compressed bitstream, the embedded decoder provides the same utility to the videoencoder. Knowledge of the reconstructed input allows the video encoderto transmit the appropriate residual energy to compose subsequentpictures.

The quantized transform coefficients for each CU are provided to aninverse quantize component (IQ) 312, which outputs a reconstructedversion of the transform result from the transform component 304. Thedequantized transform coefficients are provided to the inverse transformcomponent (IDCT) 314, which outputs estimated residual informationrepresenting a reconstructed version of a residual CU. The inversetransform component 314 receives the transform unit size used togenerate the transform coefficients and applies inverse transform(s) ofthe specified size to the transform coefficients to reconstruct theresidual values. The reconstructed residual CU is provided to thecombiner 338.

The combiner 338 adds the original predicted CU to the residual CU togenerate a reconstructed CU, which becomes part of reconstructed picturedata. The reconstructed picture data is stored in a buffer (not shown)for use by the intra-prediction estimation component 324.

Various in-loop filters may be applied to the reconstructed picture datato improve the quality of the reference picture data used forencoding/decoding of subsequent pictures. The in-loop filters mayinclude a deblocking filter 330, a sample adaptive offset filter (SAO)332, and an adaptive loop filter (ALF) 334. The in-loop filters 330,332, 334 are applied to each reconstructed LCU in the picture and thefinal filtered reference picture data is provided to the storagecomponent 318. In some embodiments, the ALF component 334 is notpresent.

FIG. 4 is a block diagram of an example video decoder configured toverify during the decoding process that the slice rates of pictures areconstrained to a maximum slice rate. The video decoder operates toreverse the encoding operations, i.e., entropy coding, quantization,transformation, and prediction, performed by the video encoder of FIGS.3A and 3B to regenerate the pictures of the original video sequence. Inview of the above description of a video encoder, one of ordinary skillin the art will understand the functionality of components of the videodecoder without detailed explanation.

The entropy decoding component 400 receives an entropy encoded(compressed) video bit stream and reverses the entropy encoding usingCABAC decoding to recover the encoded syntax elements, e.g., CU, PU, andTU structures of LCUs, quantized transform coefficients for CUs, motionvectors, prediction modes, weighted prediction parameters, SAOparameters, etc. The decoded syntax elements are passed to the variouscomponents of the decoder as needed. For example, decoded predictionmodes are provided to the intra-prediction component (IP) 414 or motioncompensation component (MC) 410. If the decoded prediction mode is aninter-prediction mode, the entropy decoder 400 reconstructs the motionvector(s) as needed and provides the motion vector(s) to the motioncompensation component 410.

The entropy decoder 400 also recovers syntax elements indicating theprofile and level used to encode the bit stream. The decoder may thencompute an expected maximum slice rate (maximum number of slices persecond) for pictures in the bit stream based on parameter values definedfor the level. Computation of the maximum slice rate is previouslydiscussed herein. The decoder may then use this maximum slice rate toverify that the slice rate for each picture is less than or equal to themaximum slice rate. In some embodiments, the decoder computes a maximumnumber of slices for each picture based on the maximum slice rateconstraint and then counts the number of slices in the picture as thepicture is decoded to verify that the number of slices is than or equalto the computed maximum slice number. The decoder computes the maximumnumber of slices for a picture (for a fixed frame rate or a variableframe rate) in the same manner as the encoder. The decoder may take anysuitable action if the number of slices exceeds the computed maximumnumber. The particular action taken by the decoder may depend on theapplication using the decoder. For example, the decoder may skipdecoding a few slices in the picture to meet real-time requirements.

The inverse quantize component (IQ) 402 de-quantizes the quantizedtransform coefficients of the CUs. The inverse transform component 404transforms the frequency domain data from the inverse quantize component402 back to the residual CUs. That is, the inverse transform component404 applies an inverse unit transform, i.e., the inverse of the unittransform used for encoding, to the de-quantized residual coefficientsto produce reconstructed residual values of the CUs.

A residual CU supplies one input of the addition component 406. Theother input of the addition component 406 comes from the mode switch408. When an inter-prediction mode is signaled in the encoded videostream, the mode switch 408 selects predicted PUs from the motioncompensation component 410 and when an intra-prediction mode issignaled, the mode switch selects predicted PUs from theintra-prediction component 414.

The motion compensation component 410 receives reference data from thestorage component 412 and applies the motion compensation computed bythe encoder and transmitted in the encoded video bit stream to thereference data to generate a predicted PU. That is, the motioncompensation component 410 uses the motion vector(s) from the entropydecoder 400 and the reference data to generate a predicted PU.

The intra-prediction component 414 receives reconstructed samples frompreviously reconstructed PUs of a current picture from the storagecomponent 412 and performs the intra-prediction computed by the encoderas signaled by an intra-prediction mode transmitted in the encoded videobit stream using the reconstructed samples as needed to generate apredicted PU.

The addition component 406 generates a reconstructed CU by adding thepredicted PUs selected by the mode switch 408 and the residual CU. Theoutput of the addition component 406, i.e., the reconstructed CUs, isstored in the storage component 412 for use by the intra-predictioncomponent 414.

In-loop filters may be applied to reconstructed picture data to improvethe quality of the decoded pictures and the quality of the referencepicture data used for decoding of subsequent pictures. The appliedin-loop filters are the same as those of the encoder, i.e., a deblockingfilter 416, a sample adaptive offset filter (SAO) 418, and an adaptiveloop filter (ALF) 420. The in-loop filters may be applied on anLCU-by-LCU basis and the final filtered reference picture data isprovided to the storage component 412. In some embodiments, the ALFcomponent 420 is not present.

FIG. 5 is a flow diagram of a method for constraining slice headerprocessing overhead during encoding of a video sequence that may beperformed, for example, by the encoder of FIGS. 3A and 3B. Initially,the level of the video sequence is determined 500. As previouslydiscussed, an encoder knows the picture size, bit-rate, frame-raterequirements of the incoming video. The encoder may use this informationto perform a table lookup in a level table such as that of FIG. 1 toselect the minimum level that satisfies the performance needs of theincoming video.

The maximum (worst case) slice rate for the level is then computed 502.Computation of the maximum slice rate for a level is previouslydescribed herein. Then, the number of slices in each picture of thevideo sequence is constrained to be less than or equal to a maximumnumber of slices computed for each picture based on the maximum slicerate. More specifically, for each picture 508, a maximum number ofslices is computed 504 based on the maximum slice rate, and the pictureis encoded 506 according to the computed maximum number of slices, i.e.,during the encoding, the number of slices in the picture is constrainedto be less than or equal to the computed maximum number of slices.Computation of the maximum number of slices for a picture for either afixed frame rate or a variable frame rate is previously describedherein.

Using the method of FIG. 5, the worst case slice header processingoverhead remains constant for a level. Taking level 5 in the table ofFIG. 1 as an example, the maximum number of slices allowed in a picturebecomes 200, 50, and 25 for 4 k×2K@30, 1080p@120 and 720p@240,respectively, and the worst case number slices per second is constantfor level 5, i.e., 6000 slices/sec, independent of frame-rates andpicture sizes.

FIG. 6 is a flow diagram of method for verifying that slice headerprocessing overhead is constrained to a maximum slice rate duringdecoding of a compressed video bit stream that may be performed, forexample, by the decoder of FIG. 4. Initially, the level of thecompressed bit stream is determined 600 by decoding an indication of thelevel from the bit stream. The maximum (worst case) slice rate for thelevel is then computed 602. Computation of the maximum slice rate for alevel is performed in the same manner as in the encoder and ispreviously described herein. Then, the number of slices in each pictureencoded in the bit stream is counted as each picture is decoded and thecount is verified against a maximum number of slices computed for eachpicture based on the maximum slice rate. More specifically, for eachpicture 610, a maximum number of slices is computed 604 based on themaximum slice rate and, as the picture is decoded 606, the number ofslices in the picture is counted. Computation of the maximum number ofslices for a picture for either a fixed frame rate or a variable framerate is previously described herein.

If the number of slices in the picture is less than or equal to 608 thecomputed maximum number of slices, processing continues with the nextpicture, if any 610, in the compressed bit stream. Otherwise, the numberof slices in the picture exceeds the maximum number. In such a case, anysuitable processing may be performed 609 to compensate for exceeding themaximum slice rate. For example, the decoding of a few slices orpictures may be skipped to meet real-time decoding requirements. Oncethe slice rate exceeded processing is complete, processing continueswith the next picture, if any 610.

FIG. 8 is a block diagram of an example digital system suitable for useas an embedded system that may be configured to constrain the slice rateper picture during the encoding process to be less than or equal to aworst case number of slices per second rate as described herein duringencoding of a video stream and/or to verify that the slice rates ofpictures are constrained to a maximum slice rate during decoding of anencoded video bit stream as described herein. This examplesystem-on-a-chip (SoC) is representative of one of a family of DaVinci™Digital Media Processors, available from Texas Instruments, Inc. ThisSoC is described in more detail in “TMS320DM6467 Digital MediaSystem-on-Chip”, SPRS403G, December 2007 or later, which is incorporatedby reference herein.

The SoC 800 is a programmable platform designed to meet the processingneeds of applications such as video encode/decode/transcode/transrate,video surveillance, video conferencing, set-top box, medical imaging,media server, gaming, digital signage, etc. The SoC 800 provides supportfor multiple operating systems, multiple user interfaces, and highprocessing performance through the flexibility of a fully integratedmixed processor solution. The device combines multiple processing coreswith shared memory for programmable video and audio processing with ahighly-integrated peripheral set on common integrated substrate.

The dual-core architecture of the SoC 800 provides benefits of both DSPand Reduced Instruction Set Computer (RISC) technologies, incorporatinga DSP core and an ARM926EJ-S core. The ARM926EJ-S is a 32-bit RISCprocessor core that performs 32-bit or 16-bit instructions and processes32-bit, 16-bit, or 8-bit data. The DSP core is a TMS320C64x+™ core witha very-long-instruction-word (VLIW) architecture. In general, the ARM isresponsible for configuration and control of the SoC 800, including theDSP Subsystem, the video data conversion engine (VDCE), and a majorityof the peripherals and external memories. The switched central resource(SCR) is an interconnect system that provides low-latency connectivitybetween master peripherals and slave peripherals. The SCR is thedecoding, routing, and arbitration logic that enables the connectionbetween multiple masters and slaves that are connected to it.

The SoC 800 also includes application-specific hardware logic, on-chipmemory, and additional on-chip peripherals. The peripheral set includes:a configurable video port (Video Port I/F), an Ethernet MAC (EMAC) witha Management Data Input/Output (MDIO) module, a 4-bit transfer/4-bitreceive VLYNQ interface, an inter-integrated circuit (I2C) businterface, multichannel audio serial ports (McASP), general-purposetimers, a watchdog timer, a configurable host port interface (HPI);general-purpose input/output (GPIO) with programmable interrupt/eventgeneration modes, multiplexed with other peripherals, UART interfaceswith modem interface signals, pulse width modulators (PWM), an ATAinterface, a peripheral component interface (PCI), and external memoryinterfaces (EMIFA, DDR2). The video port I/F is a receiver andtransmitter of video data with two input channels and two outputchannels that may be configured for standard definition television(SDTV) video data, high definition television (HDTV) video data, and rawvideo data capture.

As shown in FIG. 8, the SoC 800 includes two high-definitionvideo/imaging coprocessors (HDVICP) and a video data conversion engine(VDCE) to offload many video and image processing tasks from the DSPcore. The VDCE supports video frame resizing, anti-aliasing, chrominancesignal format conversion, edge padding, color blending, etc. The HDVICPcoprocessors are designed to perform computational operations requiredfor video encoding such as motion estimation, motion compensation,intra-prediction, transformation, and quantization. Further, thedistinct circuitry in the HDVICP coprocessors that may be used forspecific computation operations is designed to operate in a pipelinefashion under the control of the ARM subsystem and/or the DSP subsystem.

As was previously mentioned, the SoC 800 may be configured to constrainthe slice rate per picture during the encoding process to be less thanor equal to a worst case number of slices per second rate as describedherein during encoding of a video stream and/or to verify that the slicerates of pictures are constrained to a maximum slice rate duringdecoding of an encoded video bit stream as described herein. Forexample, the coding control of the video encoder of FIGS. 3A and 3B maybe executed on the DSP subsystem or the ARM subsystem and at least someof the computational operations of the block processing, including theintra-prediction and inter-prediction of mode selection, transformation,quantization, and entropy encoding may be executed on the HDVICPcoprocessors. Similarly, at least some of the computational operationsof the various components of the video decoder of FIG. 4, includingentropy decoding, inverse quantization, inverse transformation,intra-prediction, and motion compensation may be executed on the HDVICPcoprocessors.

Other Embodiments

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.

For example, in versions of HEVC after HEVC Draft 8, the concept of aslice segment is introduced. A slice segment may be a dependent slice oran independent slice, and a picture may contain both types of slicesegments. One of ordinary skill in the art will understand embodimentsin which the number of slices per second rate encompasses both dependentand independent slices.

In another example, embodiments have been described herein in which themaximum number of slices in a picture is computed as the minimum of themaximum number of slices specified for a level and a number of slicescomputed based on the maximum slice rate. One of ordinary skill in theart will understand embodiments in which rather than setting the maximumnumber of slices for a picture as the minimum of the two slice counts,the maximum number of slices per picture is set to the number of slicescomputed based on the maximum slice rate, i.e., toMaxSliceRate/FrameRate for fixed frame rate pictures andMaxSliceRate*(t(n)−t(n−1) for variable frame rate pictures.

In another example, embodiments have been described herein in which themaximum number of slices for a picture is dependent on the frame rate.One of ordinary skill in the art will understand embodiments in whichthe maximum number of slices may be computed independent of the framerate given the luma picture size of a picture. In such embodiments, themaximum number of slices for a picture may be computed as

MaxSlicesPerPicture*PicSizeInSamplesY/MaxLumaPS

where MaxSlicesPerPicture and MaxLumaPS are previously defined herein.For a level, PicSizeInSamplesY may be determined as the product of theheight and width of a luma picture and is always less than or equal toMaxLumaPS.

Embodiments of the methods, encoders, and decoders described herein maybe implemented in hardware, software, firmware, or any combinationthereof. If completely or partially implemented in software, thesoftware may be executed in one or more processors, such as amicroprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), or digital signal processor (DSP). Thesoftware instructions may be initially stored in a computer-readablemedium and loaded and executed in the processor. In some cases, thesoftware instructions may also be sold in a computer program product,which includes the computer-readable medium and packaging materials forthe computer-readable medium. In some cases, the software instructionsmay be distributed via removable computer readable media, via atransmission path from computer readable media on another digitalsystem, etc. Examples of computer-readable media include non-writablestorage media such as read-only memory devices, writable storage mediasuch as disks, flash memory, memory, or a combination thereof.

Although method steps may be presented and described herein in asequential fashion, one or more of the steps shown in the figures anddescribed herein may be performed concurrently, may be combined, and/ormay be performed in a different order than the order shown in thefigures and/or described herein. Accordingly, embodiments should not beconsidered limited to the specific ordering of steps shown in thefigures and/or described herein.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope ofthe invention.

What is claimed is:
 1. A method for encoding a picture of a video sequence in a bit stream, the method comprising: computing a maximum slice rate for the video sequence; computing a maximum number of slices for the picture based on the maximum slice rate; and encoding the picture wherein a number of slices used to encode the picture is enforced to be no more than the maximum number of slices.
 2. The method of claim 1, wherein computing a maximum slice rate comprises computing the maximum slice rate as MaxLumaSR/MaxLumaPS*MaxSlicesPerPicture, wherein MaxLumaSR is a maximum luma sample rate specified for the video sequence, MaxLumaPS is a maximum luma picture size in samples specified for the video sequence, and MaxSlicesPerPicture is a maximum number of slices per picture specified for the video sequence.
 3. The method of claim 2, further comprising: determining a level for the video sequence, wherein the level specifies values for MaxLumaSR, MaxLumaPS, and MaxSlicesPerPicture.
 4. The method of claim 1, wherein computing a maximum number of slices for the picture comprises: setting a value of the maximum number of slices for the picture as a minimum of a maximum number of slices per picture specified for the video sequence and the maximum slice rate divided by a frame rate of the video sequence.
 5. The method of claim 1, wherein computing a maximum number of slices for the picture comprises: setting a value of the maximum number of slices for the picture as a minimum of a maximum number of slices per picture specified for the video sequence and the maximum slice rate multiplied by a difference in display time between the picture and a picture immediately preceding the picture in display order.
 6. A method for encoding a picture of a video sequence in a bit stream, the method comprising: determining a level for the video sequence; computing a maximum slice rate for the video sequence as MaxLumaSR/MaxLumaPS*MaxSlicesPerPicture, wherein MaxLumaSR is a maximum luma sample rate specified for the level, MaxLumaPS is a maximum luma picture size in samples specified for the level, and MaxSlicesPerPicture is a maximum number of slices per picture specified for the level; computing a maximum number of slices for the picture as a minimum of MaxSlicesPerPicture and the maximum slice rate divided by a frame rate of the video sequence when the frame rate is fixed; computing a maximum number of slices for the picture as a minimum of MaxSlicesPerPicture and the maximum slice rate multiplied by a difference in display time between the picture and a picture immediately preceding the picture in display order when the frame rate is variable; and encoding the picture wherein a number of slices used to encode the picture is constrained to be no more than the maximum number of slices.
 7. An apparatus configured to encode a picture of a video sequence in a bit stream, the apparatus comprising: means for computing a maximum slice rate for the video sequence; means for computing a maximum number of slices for the picture based on the maximum slice rate; and means for encoding the picture wherein a number of slices used to encode the picture is enforced to be no more than the maximum number of slices.
 8. The apparatus of claim 7, wherein the means for computing a maximum slice rate computes the maximum slice rate as MaxLumaSR/MaxLumaPS * MaxSlicesPerPicture, wherein MaxLumaSR is a maximum luma sample rate specified for the video sequence, MaxLumaPS is a maximum luma picture size in samples specified for the video sequence, and MaxSlicesPerPicture is a maximum number of slices per picture specified for the video sequence.
 9. The apparatus of claim 8, further comprising: means for determining a level for the video sequence, wherein the level specifies values for MaxLumaSR, MaxLumaPS, and MaxSlicesPerPicture.
 10. The apparatus of claim 7, wherein the means for computing a maximum number of slices for the picture sets a value of the maximum number of slices for the picture as a minimum of a maximum number of slices per picture specified for the video sequence and the maximum slice rate divided by a frame rate of the video sequence.
 11. The apparatus of claim 7, wherein the means for computing a maximum number of slices for the picture sets a value of the maximum number of slices for the picture as a minimum of a maximum number of slices per picture specified for the video sequence and the maximum slice rate multiplied by a difference in display time between the picture and a picture immediately preceding the picture in display order. 